The first of the above positions, the different capa eities for heat of the air before and after it had undergone the change which it experiences in combustion or in respiration, constituted, as is well known, the main point to which Crawford directed his elaborate train of experiments, and forms the basis of his theory of inflammation generally, of which animal heat composes only one example. With respect to the second position, the different capacities of arterial and venous blood, Crawford made this the subject of a long and careful examination, and although there are many parts in it of very delicate execution, and which required a minute attention to various circumstances that might interfere with the results, yet he appears to have been so well aware of the sources of error, and to have so carefully guarded against them, that, I think, a perusal of the experiments can scarcely fail to impress the mind with a conviction of their general truth. The conclusion which Crawford deduced was, that the specific heat of arterial, was greater than that of venous blood in an average proportion of 114.5 to 100. If we admit the fact, the conclusion is obvious and most important. It follows that when the blood is converted from the venous to the arterial state, it renders latent a part of the heat which would otherwise have been liberated by the union of the oxygen and the carbon, and would have raised the temperature of the blood in the pulmonary vessels. The heat therefore appears to be employed in three ways; a part of it in counteracting the effect of the cold air which is taken into the lungs,

another portion in producing the vapour that is expired, while the remainder supplies the arterial blood with what is requisite, in consequence of its encreased capacity, to support its temperature at the same degree with that of the venous blood.

It would appear that these operations are so nicely adjusted to each other, that the temperature of the blood in the different parts of the body is kept nearly at a uniform standard, and especially, that while this fluid passes through the lungs, and in conjunction with the air, undergoes that change which serves as the actual origin of the heat of the animal, its temperature is little, if at all affected, the heat which would otherwise be liberated being the whole of it employed in the three ways mentioned above. We have here a remarkable example of that nice adaptation of the different operations of the living body to each other, which forms so distinguishing a feature of the animal œconomy, and in which it so infinitely surpasses any contrivances of a merely mechanical or physical nature. Admitting the truth of the theory it will follow, that the more carbon is separated from the blood and united to oxygen, the more heat will be disengaged, the more completely therefore will the blood be arterialized, and consequently the more will its capacity be encreased, and the greater quantity of heat will it require to maintain its temperature. According to Crawford's doctrine the blood is not warmed in passing through the lungs, although this is the organ in which it acquires its heat, but it is in the capillary part of the systemic circulation, when

the blood again becomes venalized, that the heat is liberated. This effect therefore takes place in that part of the body where it is the most necessary to counteract the effect of the surrounding medium, which is, in most cases, colder than the body itself, and where by its diffusion over a great extent of surface, it must tend to make the heat nearly uniform through the whole of the system.

After the publication of Crawford's theory, Lavoisier, in a subsequent memoir, recurred to the subject of animal heat. He still maintained his former opinion, that it is derived from the union of carbon and oxygen in the lungs, and that respiration is, in every respect, analogous to the process of combustion. He assumes that all parts of the body are preserved at the same temperature, and with respect to the manner in which the heat is equalized, he observes that it depends upon the three following causes; 1. Upon the rapidity of the circulation of the blood, by which the heat that is acquired in the lungs is quickly transmitted to all parts of the body; 2. Upon the evaporation which takes place from these organs, which carries off a part of their heat; and 3. From the encreased capacity for heat, which blood acquires, when it is converted from the venous to the arterial state. It will be observed that this view of the subject, so far as animal temperature alone is concerned, is strictly conformable to the doctrine of Crawford.3

3 Mem. Acad. Scien. pour 1780, p. 406. We may remark, however, that Lavoisier, in one of his subsequent papers, speaks of the combustion as taking place in the lungs, although he adds,

Lavoisier, however, as is too frequently the case, does not introduce the name of Crawford, or even allude to his experiments on the different capacities of arterial and venous blood, and the same silence is observed on this point, when, in another part of this paper, he compares his own hypothesis with that of Crawford respecting the heat generated by combustion, and notices the experiments on the different capacities of oxygen and carbonic acid. Lavoisier seems to have maintained the same opinion respecting animal heat in his future researches; he always speaks of it as a case of combustion, and supposes that the heat is generated upon the same principle, as in the formation of carbonic acid by the more rapid union of oxygen and carbon.

Although Crawford's theory so admirably accounted for all the phenomena, and appeared to be so strictly deduced from facts and experiments, yet the extreme delicacy of the operations on which it rested, as well as the interest excited by its great importance, led to a rigid examination of all its fundamental positions ; the consequence of which was that they have all of them been controverted, and we have an account of a number of experiments, the results of which are reported as being directly opposite to those of Crawford, or at least, as not warranting his conclusions. The main points on which the theory is supported are the following; that heat is extricated when oxygen is

"and perhaps in other parts of the system;" Mem. Acad. Scien. pour 1790, p. 601.

4 Mem. Acad. Scien. pour 1780. p. 394.

converted into carbonic acid, in consequence of the diminished capacity of the latter, that the capacity of arterial is greater than of venous blood, and that the blood in the two sides of the heart, and in the large trunks of the pulmonic system possesses the same temperature. With respect to the first of these points, the different capacities of oxygen and carbonic acid, it must be considered as a question which affects the theory of combustion generally, and in which the cause of animal temperature is not particularly concerned. When carbon is united to oxygen so as to produce carbonic acid, we find that heat is liberated, and this takes place not merely in that rapid union of the bodies, which may be considered essential to a proper combustion, but in the slow combination of them, such as occurs in fermentation,

putrefaction, and germination." In whatever way

therefore we account for the liberation of heat in these processes, we may employ the same method to explain the heat generated by respiration. We may even go farther, and say, that as heat is always liberated when this union of oxygen and carbon takes place, the same effects must necessarily follow from the production of carbonic acid in respiration. The difficulty therefore is not to account for the heat produced by respiration, but to remove the objection which was originally urged against Black's hypothesis, that the temperature of the blood in the lungs ought

5 In the malting of barley, which is a case of germination, we are informed by Dr. Thomson, that the temperature of the grain is raised as much as 10°; Chem. v. iv. p. 374.